Crab Nebula in Context

X-ray image of Crab Nebula
NASA/CXC/SAO

The explosion that created the Crab Nebula was described by Yang
Wei-te, the court astronomer (astrologer) to the Chinese Sung
emperor, and may be the basis of two petroglyphs found in the US
southwest. We now see the Crab as it appeared 945 years after
the supernova. Its name derives from the resemblance of the
optical nebula to a claw of a crab.

Neutron stars are extremely dense, with a mass slightly greater
than the Sun compressed into a sphere only 20 km across. The
strong gravity of the star packs the neutrons in its core more
tightly than neutrons are packed in the nucleus of an
atom.

Every 33 milliseconds the neutron star in the Crab spins one
full revolution. This means that a point on the equator of the
neutron star travels at roughly 4 million miles per hour. The
star was spun up when it collapsed from a normal, large and
relatively slowly rotating, star down to the compact size of the
neutron star. The effect is the same as when a twirling ice
skater pulls in her arms and spins faster.

Hubble Space Telescope optical
image of inner part of Crab Nebula

The neutron star's rapid rotation (30 times/second) powers the
radiation we see from the Crab pulsar and the nebula. The
neutron star has a strong magnetic field, 10 trillion times as
strong as the magnetic field of the Earth. Because the star is
rotating, the magnetic field acts like a gigantic electric power
generator. The Crab neutron star produces a total power output
equal to 100,000 times the power output by the Sun. Generating
this power gradually slows down the pulsar; the period of
rotation of the Crab pulsar is increasing by 15 microseconds per
year.

This stellar dynamo also produces a wind of electrons and
positrons, the anti-matter partner of the electrons. The
positrons are created in the intense electrical fields of the
generator at a rate of 10^37 positrons per second. The electrons
and positrons flow out from the Crab at close to the speed of
light and energies up to at least one trillion electron
volts.

There are approximately 1000 known pulsars and the number increases almost
daily. Most are discovered by radio observations. The Crab is one of the
youngest and most energetic pulsars known. A few dozen pulsars are also
seen to pulse in X-rays and six are seen to pulse in gamma-rays. The Crab
has been seen to pulse in almost every wavelength, in radio, optical, X-rays,
and gamma-rays. The nebula is also visible over this broad range of wavelengths.